1. Field of the Invention
The present invention relates generally to power supplies for a cold-cathode fluorescent lamp ("CCFL") that may be used for back lighting a display of a portable device, and, more particularly, to a switched-mode inverter circuit which uses a capacitor to reset a transformer's core, and uses its magnetizing inductance to store and transfer energy during negative half-cycles.
2. Description of the Prior Art
Similar to a conventional hot-cathode fluorescent lamp ("FL") used for office and home lighting, CCFLs are high-efficiency, long-life light sources. By comparison, incandescent lamps have efficiency in the range of 15 to 25 lumens per watt, while both FLs and CCFLs have efficiency in the range of 40 to 60 lumens per watt. Furthermore, the average life of an incandescent lamp is only about 1,000 hours. However, FLs and CCFLs, on average, last for 10,000 hours or more.
The main difference between a hot-cathode FL and a CCFL is that the CCFL omits filaments that are included in a FL. Due to their simpler mechanical construction and high efficiency, miniature CCFLs are generally used as a source of back lighting for Liquid Crystal Displays ("LCDs"). LCDs, whether color or monochrome, are widely used as displays in portable computers and televisions, and in instrument panels of airplanes and automobiles.
However, starting and operating a CCFL requires a high alternating current ("ac") voltage. Typical starting voltage is around 1,000 volts AC ("Vac"), and typical operating voltage is about 600 Vac. To generate such a high ac voltage from a dc power source such as a rechargeable battery, portable computers and televisions, and instrument panels, include a dc-to-ac inverter having a step-up transformer.
Presently, the majority of CCFL inverter circuits are based on a circuit generally known as the current-fed Royer circuit. A Royer circuit, referred to by the general reference character 10 in FIG. 1, may include two NPN bipolar transistors 12 and 14 together with a saturable-core transformer 16. Collectors 18 of each of the transistors 12 and 14 connect respectively to opposite ends of a primary winding 22 of the transformer 16. A center-tap 24 of the transformer 16 connects to a positive terminal 26 of a battery 28. Emitters 32 of the transistors 12 and 14 connect in parallel to circuit ground of the Royer circuit 10, to which a negative terminal 34 of the battery 28 also connects. A feedback secondary winding 36 of the transformer 16 connects between bases 38 of the transistors 12 and 14. A bias resistor 42 connects between the base 38 of the transistor 12 and the positive terminal 26 of the battery 28. A CCFL 44 and a decoupling capacitor 46 connect in series across a power output secondary 48 of the transformer 16. The non-linear current-gain characteristics of the bipolar transistors 12 and 14 in conjunction with the non-linear permeability of the saturable-core transformer 16 cause the Royer circuit 10 to be self-oscillating. Consequently, the Royer circuit 10 omits any external clock or driver circuit for the transistors 12 and 14.
The Royer circuit 10 is basically a fixed voltage inverter. That is, the Royer circuit 10 steps-up voltage with a constant ratio that is proportional to the number of turns on the secondary winding divided by the number of turns on the primary winding. Consequently, the Royer circuit cannot maintain a constant output voltage if input voltage or load current varies. Therefore, a regulator circuit is generally used to supply electrical power to the Royer circuit. The regulator circuit, normally a switch mode step-down converter, delivers constant input power to the Royer circuit so the output load, e.g. a CCFL, receives constant electrical power.
FIG. 2 depicts a typical current-fed Royer circuit combined with a regulator circuit. Those elements depicted in FIG. 2 that are common to the Royer circuit depicted in FIG. 1 carry the same reference numeral distinguished by a prime ("'") designation. The regulator circuit depicted in FIG. 2 includes a PNP power control transistor 52, a free-wheeling diode 54, an inductor 56, a current-sensing resistor 58, and a switching regulator controller 62. An emitter 64 of the power control transistor 52 connects to the positive terminal 26' of the battery 28'. A collector 66 of the power control transistor 52 connects in series with the inductor 56 to the center-tap 24' of the primary winding 22' of the transformer 16', and to a cathode 68 of the free-wheeling diode 54. An anode 72 of the free-wheeling diode 54 connects to circuit ground. The current-sensing resistor 58 connects in series between the emitters 32' of the transistors 12 and 14 and circuit ground. A current-sensing input-terminal 74 of the switching regulator controller 62, which may be a LT1182 or a LT1183 CCFL/LCD Contrast Dual Switching Regulator integrated circuit ("IC") marketed by Linear Technology of Milpitas, Calif., connects to a juncture between the emitters 32' and the current-sensing resistor 58. A power-input terminal 76 of the switching regulator controller 62 connects to the positive terminal 26' of the battery 28'. An output terminal 78 of the switching regulator controller 62 connects to a base 82 of the power control transistor 52 to alternatively first turn the power control transistor 52 on, and then turn the power control transistor 52 off.
Since the Royer circuit 10' depicted in FIG. 2 uses two stages of power conversion, i.e. the regulator connected in series with the current-fed Royer circuit of FIG. 1, its electrical efficiency is comparatively low, i.e. approximately 70-80%. Since LCD back lighting consumes a significant amount of electrical power in portable computers and televisions, i.e. approximately 20% to 30%, excessive power consumption by the Royer circuit significantly reduces the amount of operating time provided by a fully charged battery. Furthermore, the transformer 16 or 16' requires four windings, two of which connect in series to provide the center-tap 24 or 24' for the primary winding 22 or 22'. Because of the four-winding structure, and the high voltage generated across the power output secondary 48 or 48', the transformer 16 or 16' is comparatively difficult to manufacture, and is prone to arcing failures.